The present invention relates to data transfer and more particularly, to a control method of data transfer among a plurality of central processing units (CPUs).
In conventional methods of data transfer among a plurality of CPUs, it has been so arranged, for example, that dedicated data transfer lines LA, LB, LC, LD and LE are provided between the two corresponding CPUs of a No. 1 CPU A, a No. 2 CPU B, a No. 3 CPU C and a No. 4 CPU D as shown in FIG. 1. However, the known arrangement of the above described type has such disadvantages that an increasing number of dedicated data transfer lines are required to be provided as CPUs are increased in number, resulting in complicated data transfer, and rise in the hazard rate and cost.
Furthermore, in the case where a printer is provided with various external terminals such as a plurality of a cathode ray tube (CRT), a keyboard, etc. each of which is connected with a CPU through an interface and more particularly, a printer of a line scanning type such as a laser beam printer is provided with a character generator, a video terminal, a CRT, etc. each of which is connected with a CPU through an interface or a relatively small-sized bus connection is formed by, for example, providing terminals in the housing of the printer, the conventional method of the above-described type has such inconveniences as necessity of dedicated ICs, complicated protocol, difficulty in synchronization of operations of output units, increase in the cost etc.